(1) Field of the Invention
This invention relates to a hot press suitable for use in producing a multilayer printed circuit board by alternately arranging a plurality of substrates and prepregs in superposed relation and applying pressure thereto while heating them.
(2) Description of the Prior Art
Some printed circuit boards (hereinafter simply boards) have very complex wiring patterns and some have wiring patterns of very high density because of the need to meet the requirements of providing patterns of complex wiring connections having fixed components. Thus, some of them are impossible to produce as a single board. In this case, it is usual practice to split the wiring pattern into a plurality of groups and print the wiring pattern groups on respective substrates. Then the substrates are superposed one over another in a predetermined order and pressed together, before the wiring pattern groups are electrically connected together, to provide a single board which is usually referred to as a multilayer printed circuit board (hereinafter multilayer board). To produce a multilayer board, substrates each having one of the wiring pattern groups printed thereon are pressed together by means of a hot press.
A hot press comprises a pair of bolsters or an upper bolster and a lower bolster, a pair of heating plates located in a space defined between opposing surfaces of the upper and lower bolsters and each disposed adjacent one of the opposing surfaces of the bolsters through a heat insulating member interposed therebetween, and a plurality of passages each formed in one of the heating plates for supplying steam thereto for heating purposes and a plurality of passages each formed in one of the heating plates for supplying water thereto for cooling purposes.
Substrates and prepregs are alternately arranged between the heating plates in superposed relation and held between them, and pressure is applied thereto while they are heated.
The hot press of the aforesaid construction suffers the disadvantage that even if the heating plates are provided with opposing surfaces which are parallel to each other, the multilayer board produced tends to have a thickness which is greater in the central portion than in the peripheral portion.
Tests were conducted by alternately arranging five substrates [each having the dimensions of 500 mm.times.500 mm.times.0.2 mm (thickness) and formed of glass fiber reinforced epoxy resin] and eight prepregs [each having the dimensions of 500 mm.times.500 mm.times.0.1 mm (thickness)] in superposed relation with two prepregs being interposed between the two substrates or in four positions in the assembly of substrates and prepregs which were held between the heating plates heated to a predetermined temperature and pressed together with a pressure reaching a maximum of about 45 tons, to provide a multilayer board. A test showed that the thickness of the multilayer board was 1.78 mm (maximum) in the central portion and 1.6 mm (minimum) in the peripheral portion, indicating that the single multilayer board has a difference of 0.18 mm between the maximum and minimum values of its thickness. This difference will hereinafter be referred to as a variation in thickness.
There is a correlation between the variation in thickness from one portion of the multilayer board to another and the magnitude of a difference between the positions of the substrates which constitute the multilayer board (which will hereinafter be referred to as a displacement of one layer from another). Thus, the smaller the variation in thickness, the smaller becomes the displacement of one layer from another. Consequently, it is desired that the variation in the thickness of a multilayer board be minimized.
To meet the requirement of minimizing the variation in thickness, one would conceive of constructing the heating plates of the hot press in such a manner that the space defined between their opposing surfaces has a smaller distance between central portions of the opposing surfaces than between peripheral portions of the opposing surfaces.
However, when the heating plates of this construction were used, it would be necessary to alter the shape and configuration of the heating plates (the size of a protuberance in the central portion as compared with the flatness of the peripheral portion, for example) depending on the size, thickness and number of the substrates and prepregs constituting the multilayer board. This would make it necessary to have on hand different types of heating plates matching the types of the multilayer board. Thus, when the multilayer boards to be produced are of many different types but the number of each type produced is small, production and management of the heating plates would be time-consuming and causes an increase in cost. In addition, a large space would be required for storing them for use when necessary.